In general, vehicle drivelines transmit torque from a vehicle's engine to its wheels. Automotive drivelines, such as all-wheel drive (AWD) drivelines, commonly employ final drive units (FDUs) for transmitting torque to left and right sideshafts that are located downstream of a vehicle engine and transmission. For example, FDUs may receive driven torque from a propshaft. Final drive units can be mounted at the rear of an automotive driveline or at the front, depending on the architecture of the particular driveline and the location of its engine and transmission. Moreover, an FDU may be provided at both the front and rear, e.g., where one drives the front wheels and the other drives the rear wheels in an AWD driveline. FDUs typically include differential gearsets that allow wheels on one sideshaft to spin faster or slower than wheels on the other sideshaft.
Vehicle drivelines may be capable of controlling and varying a torque apportionment between axles or other output shafts of a driveline. Merely as one example, a transfer case or Power Take Off (PTO) unit may apportion torque between front and rear axles of a vehicle. In some known approaches, a driveline may reduce a wheel or axle's share of engine torque when slip is detected at that wheel/axle.
Torque management of the vehicle that is appropriate for one set of operating conditions may harm performance in other conditions. Thus, since vehicles may not be capable of robustly detecting all of the different operating conditions influencing vehicle performance, vehicle manufacturers typically must accept performance compromises. For example, hill or grade climbing abilities of a vehicle may be enhanced by increasing an amount of torque sent to a secondary drive axle while traversing the hill. However, the same reaction during a sharp turn on a slippery or low-friction surface (e.g., snow, ice, gravel, etc.) could reduce vehicle stability and could therefore be undesirable. Thus, vehicle manufacturers sometimes sacrifice the ability to maximize hill climbing capabilities in the name of more stable driveline behavior on low-friction surfaces. Moreover, available methods for determining when a vehicle is traversing a graded surface may not accurately determine the presence of a grade and/or the magnitude thereof.